Apparatus and method for making prestressed concrete tubular members



July 31, 1962 0.. E. OLIVIER 3,046,631

APPARATUS AND METHOD FOR MAKING PRESTRESSED CONCRETE TUBULAR MEMBERS Filed Dec. 11, 1957 2 Sheets-Sheet 1' I lNVENTQR, Da/mwlE,

ATTORNEY July 31, 1962 D. E. OLIVIER 3,046,631

APPARATUS AND METHOD FOR MAKING PRESTRESSED CONCRETE TUBULAR MEMBERS 2 Sheets-Sheet 2 Filed Dec. 11. 1957 INVEN EOR Duruel E, (Th/two United btates Fascist 3,046,631 APPARATUS AND METHOD FQR MAKING PRE- STRESSED CONCRETE TUBULAR MEMBERS Daniel E. Olivier, Bedminster, N.J., assignor to Lock Joint Pipe Company, East Orange, N.J., a corporation of New Jersey Filed Dec. 11, 1957, Ser. No. 702,035 6 Claims. (Cl. 25-118) This invention relates to the manufacture of prestressed concrete tubular bodies, such as pipes, and to apparatus and procedures for placing and embedding longitudinally extending highly tensioned high tensile steel wires within the wall of a prestressed concrete pipe.

A concrete pressure pipe of the type with which the present invention is concerned comprises a tubular concrete member or hollow core around which a tensioned wire is wound. The tensioned wire winding strengthens the core and enables the core to withstand high internal fluid pressures. At first impression it would appear that the bursting strength and the impermeability of such a core would be limited only by the contracting force which can be supplied by a tensioned wire winding and the strength of the circumferentially compressed concrete. This is not the case because cracks can occur in the concrete during the application of the tensioned wire winding and also during the operation of the pipe after it has been placed in the ground, unless the concrete is compressed longitudinally by sorne other reinforcing such as longitudinally disposed tensioned rods or wires. Consequently, for containing higher fluid pressures the longitudinal compressive stress in the concrete which is imparted by tensioned longitudinals as well as the circumferential contracting force of a tensioned wire winding must be increased in proper relationship.

A high order of longitudinal compression can be obtained by the use of prestressed wires of high tensile steel embedded in the concrete core. These wires are strung through a mould for forming the core and are highly tensioned before the mould is filled with concrete. It has been a practice to hold the wires in tension by anchorages located outside and beyond the ends of a mould. Before the hardener core could be removed from the mould, the wires had to be cut betwen their anchorages and the mould ends in order to enable the removal of the mould ends from the moulded core. After the ends of the mould had been removed, it was necessary again to cut the wires to remove the ends of the wires protruding from each end of the core. In carrying out the second cutting operation, manufacturers have usually been content with cutting the wires close to or flush with the end faces of a concrete core and with giving the exposed ends of the wires a modicum of protection by brushing them with a bi-tumastic paint or similar protective material.

The present invention reduces the number of cutting operations previously required to free the mould ends from the compressed core and to leave the core free of protruding wires. It also results in leaving but a short cylindrical bore in the end of the concrete of approximately the same diameter as that of the wire and which is more suited for receiving and holding a solid plug of a durable material capable of protecting the wire for a considerable time.

Individually manipulatable anchoring devices of the mould of the present invention secure the ends of the tensioned wires and hold them in place in the mould. The anchoring devices press on the mould ends and the thrusts of the mould ends are counteracted by the intervening shell or main casing which forms the exterior of the core. Each anchoring device is constructed and supported in a manner to enable its rotation about the axis of a gripped wire so that the wires can be broken off within the concore when the last of the several ends of wires extending therethrough has been severed by rotation of its anchoring device.

The break-off points of the wires are located before the wires are stretched and assembled in a mould. The break-off points are established by indentations which are produced by the lateral application of pressure with a tool having rounded edges or surfaces. The pressure exerted by the tool displaces the longitudinally extending fibers of a wire by deformation and without cutting. A deformation thus produced in a high tensile steel wire decreases torsional resistance considerably as compared with only little less of tensile strength. This is extremely important because of the high cost of the high grade steel wires of the character required for prestressing concrete pipes and the desire to make economical use of them. Even though a Wire is highly stressed in tension an end protruding from the outside of a mould end can be broken off at a breal -off point simply by rotating the anchoring device.

The break-off points are indented in the unstretched wire at such a spacing as to locate them where the wires are to terminate when they are stretched and embedded in a finished pipe.

The spacing or distance between the break-oft points along a tension-free wire is determinable by the formula: L-2pd wherein:

L stands for the length of the pipe mould between the inside faces of the mould ends (the length of the pipe); d stands for the distance of a terminal end of the wire from the nearest end face of the pipe;

p stands for the tensile stress intensity to be applied to the wire, and r E stands for the modulus of elasticity of the wire.

It is apparent that the spacing S is constant for all pipes having the same specifications.

When the wires have been strung through the mould cavity one end of each Wire is anchored to the mould end by the individual anchoring devices with an indentation or break ofi point located at the desired distance from the inside face of the mould end. Tension is then applied to the other end of the wire until the wire is stretched sufficiently to locate the secondv indentation or break-01f point at the same distance from the inside facev of the mould end at the other end of the mould, or until the desired predetermined tension in the wire has been attained, and then said other end of the wire is anchored to said other end of the mould. The wires may be tensioned one or more at a time. The lengths of the tensioned Wires within the mould are wholly embedded in concrete as the concrete is placed and the pipe is moulded. A.

dense concrete in bonded relationship with the wires is obtained by rotating the mould in a roller suspension machine or in a centrifugal machine as is well understood.

Although the novel features which are believed to be characteristic of this invention will be particularly pointed out in the claims appended hereto, the invention itself, as to its objects and advantages, and the manner in which it may be carried out, may be better understood by referring to the following description taken in connection with the accompanying drawing forming a part hereof, in which:

FIG. 1 is a general outside view of a rotatable mould with one end partially cut away and showing tensioned wires extending longitudinally through the moulding space;

FIGS. 2 and 3 are representative of prepared lengths of a high tensile steel wire in an unstrained or tensionless condition andillustrating, respectively, the wire as viewed in planes at right angles to one another;

FIG. 4 diagrammatically illustrates the relationship of a strained and tensioned wire in reference to the length of the moulding space or of a concrete pipe;

FIG. 5 is a longitudinal section through the wall of a mould and showing a placed wire before tensioning;

FIG. 6 illustrates the tensioning and anchoring of one end of a wire;

FIG. 7 illustrates an anchored end of a wire prior to the placement of concrete;

FIG. 8 is illustrative of the moulding and hardening stages;

FIG. 9 shows the mould shell removed and the stage preceding severance of the wire and the freeing of the mould end from an end of the formed core of concrete;

FIG. 10 is an end view of part of a mould end and showing means for rotating an, anchoring device to twist the end of an embedded wire;

FIG. 11 shows the concrete core after the mould end has been removed;

FIG. 12 is a longitudinal section of a combined anchoring and twisting-off device; and

FIG. 13 is a cross-section on line 13-13 of FIG. 12.

In the drawing, certain specific disclosure of the invention is made for purposes of explanation, but it will be understood that the details may be modified in various respects without departure from the broad aspect of the invention.

In the following description and in the claims, various details will be identified byspecific names for convenience, but they are intended to be as generic in their application as the art will permit.

The mould illustrated in the accompanying drawing includes 'a shell formed of two semi-circular shell plates 10 'and711 to which mould ends 12 and 13 are fitted. The mould ends are in the form of rings. The longitudinal edges of each shell plate are provided with flanges 14 and 15 by which the shell plates are bolted together to form the outer periphery of a tubular moulding cavity. A flange 14 welded to the shell plate 10 and a flange 15 welded to the shell plate 11 are shown in FIG. 1 and a similar set of flanges are located oppositely across the mould. Removable devices such as a series of bolts 16 secure the shell plates together. Ears 17 provide means whereby attachment to the mould may be made for carrying purposes.

The semi-circular segments 18 and 19 attached to the respective shell plates adjacent one end of the mould stiffen the curvature of the plates and are adapted for use as a rolling ring. A similar pair of segments 20 and 21 are located at the other end of the mould.

The mould end ring 12 is centered and supported with respect to the shell within a socket formed by lips 22 and 23 which extend from the respective shell plates. The mould end ring 12 has a bevelled surface 24 which abuts a similarly sloped surface adjacent the end of the shell when the mould is assembled and ready for use. The inner surface 25 of the mould end ring moulds the end face of aconcrete core or pipe and its remaining interior may be shaped to provide the shape desired to begiven to the end of any particular tubular body of concrete. The thickness of the wall of concrete is determined by the diameter of the inner surface 26 of the mould end ring.

For forming a tubular core for a straight-walled pipe or for a pipe having spigot ends at both ends the two ends of the shell and the rings at either end of the mould are constituted alike. In the embodiment of the mould illustrated in FIG. 1, the ends of the mould are mirror likenesses of one another (FIG. 5). Each mould end ring is "solid and suificiently stout to sustain without dis- 4 tortion the cumulative pull of all of the tensioned wires 27 connecting between them.

The wires 27 are equally spaced circumferentially around the mould in such number and at such intervals as to uniformly distribute the longitudinal compression around the concrete core to be cast in the mould. The wire size, with accompanying stress intensity, and the number and spacing of wires are determined to develop the longitudinal compressive stress desired to be attained in a particular concrete core.

The ends of the wires are similarly anchored to the mould end rings 12 and 13. Each of the mould end rings is drilled as at 29 (FIG. 5) at proper intervals for the passage of the several wires. The outer side of each ring is provided with a bore 30 concentric with each drilled hole for receiving a washer 31 and the cylindrical nose 32 of an anchoring device 33. The anchoring device acts as a clamp which can be slid along a wire for bringing the device into abutting engagement with the washer against the end ring.

Referring to FIGS. 12 and 13, each of the anchoring devices has a nut portion 35 whereby the device can be rotated with a wrench for twisting an end of a wire. The hollow interior of the device has a smooth tapered surface 36 which narrows towards the free end of the nose 32 and opens at its larger end into a chamber 37 which houses a coiled spring 39. A retaining ring 40, which is buttressed by a plurality of lugs 41, engages one 'end of the spring. The other end of the spring presses upon the larger ends of a nest of three similar jaws 42.

Each jaw 42 is in the form of a segment of a trim cated cone having exterior surfaces 43 corresponding in curvature to the tapered surface 36 on the interior of the anchoring device and a serrated interior providing teeth 44 for gripping a wire. The three jaws are individually interengaged with a ring 45 in a manner to limit axial displacement of the jaws with respect to one another without interfering with their adjustability. The construction is such that the teeth on the jaws bite into a wire and prevent relaxation of tension on the wire when the device is held against movement as by a mould end. The teeth efiect such a solid grip on the wire as to enable the gripped portion of the wire to be twisted notwithstanding a very high tensile stress on the wire.

In preparing the mould for casting one of the anchoring devices 33 and a washer are slipped over a wire as shown at the left of FIG. 5 and the wire is threaded through aligned drilled holes 29 in the end rings 12 and 13. Another anchoring device and washer are then slipped onto the end of the wire as shown at the right of FIG. 5, preferably with the end of the wire flush with the outer end of the device.

In order to provide twist-off points of lower torsional strength with the least loss of tensional strength the wires are provided with rounded indentations at their twistoif points. As illustrated in FIG. 2, these may be produced by applying pressure to the wire from opposite sides by a pair of parallel carbide tipped jaws 47, 48 which compact the steel fibers and leave rounded indentations 49, 50. Highly stressed high tensile steel wires are critical to secondary stress concentrations and caution is taken to prevent scratches or nicks occurring in the wires. In practice, a radial reduction in a Wire of slightly less than twenty-five percent of its diameter and the use of jaws having compacting surfaces rounded on a radius of one-sixteenth of an inch have worked out satisfactorily, but these proportions may be varied to suit different sizes of wires and other conditions prevailing in the manufacture of pipes of different sizes and strengths.

In a finished pipe the twist-off points are located a distance d (FIG. 4) inward from the ends of a concrete core, say within a range of about A" to /2" from an end of the mould, for example, although the actual location selected will vary depending upon the size of the wires and the length and diameter of a core. Once the ultimate locations of the twist-oft points are settled, the interval S along a longitudinally unstressed wire may be calculated as hereinabove explained and the indenting jaws 47, 48 and 51, 52 are positioned for impressing the notches. All of the wires needed can be indented Without changing the spacing between the two pairs of jaws.

After the wires have been placed in the mould, FIG. 5, the predetermined stress intensity p is applied to the wire by a tensioning machine 53 (FIG. 6). The right end of the wire remains anchored with respect to the mould end 13, as shown in FIG. 5, as the wire is stretched. When the desired stress is attained the washer 31 and the second anchoring device 33 are moved into engagement with the mould end 12, FIG. 6. The stretched Wire is firmly anchored as the tensioning machine is removed, FIG. 7, and then the unused Wire is cut to release it from the mould.

After all of the wires have been tensioned and anchored in like fashion the mould is rotated and filled with a relatively dry and rich concrete mix 54, FIG. 8. In view of the quality of the concrete and the moulding method employed the concrete is 'bonded to the wires throughout their lengths.

When the placement of the concrete is completed, the filled mould is removed from the rotating machine and the concrete is cured for adequate strength. Upon completing the curing the shell plates 10 and 11 are stripped from the hardened core 55, FIG. 9, thereby transferring to the solid concrete and mould ends the reaction to the tensioned wires which was previously provided by the shell.

The end rings are then freed from the tension of the wires by removing the anchoring devices. As shown in FIG. 10, the anchoring devices may be removed by the use of a wrench 55 whereby the end of a wire is broken off at the adjacent twist-off point simply by rotating the wrench through a moderate angle of the order of 70 more or less. The removal of the loosed wire ends leaves a relatively small cylindrical bore adapted to be solidly plugged with cement mortar 57 to protect the wire end from rusting.

The wires used may be plainor lightly dirnpled at intervals along their lengths, When plain wires are employed, it is desirable that they be provided with flattened portions d1 (FIGS. 2, 3 and 4) to the sides of the notches which are the more remote from the ends of the mould. The flattened portions prevent rotation of the ends or" the wires which remain in the core. These can be applied by dies at the same that the bre -off notches are indented.

Though popularly regarded as a rigid material concrete which is subjected to a sustained load has a certain degree of plasticity aside from drying shrinkage. Inelastic strains including creep strain due to plastic flow develop under continuous load over a prolonged period of time and these strains are related to theamount of compression applied to the concrete.

The prestressing technique requires that only wires of very high tensile stress he used. Since the crack-proof quality of a prestressed concrete pipe is a function of the amount or" pre-compression the concrete is subjected to, the initial tensile stress in each wide should be above 100,000 pounds per square inch to make sure that the subsequent inelastic losses or strain in the concrete will become a small percentage of the total strain induced in the wire. Unless this is done, the ultimate inelastic strain of the concrete may completely counteract or negative the effect of the initially induced compression with little or nor resulting prestressing improvement. For example, a wire having a tensile strength of approximately 200,000 pounds per square inch may be required to be used in order to produce a resultant tensile stress intensity in the wire of approximately 100,000 pounds per square inch. Quality wires with uniform chemical and physical properties are required. Hard drawn steel spring wire is highly suited for the purpose.

The manner of employing the invention will be apparent to those skilledin the art in view of the foregoing disclosure. While the described apparatus illustrates a manner and means for carrying out the invention, it is understood that some of the various features and elements thereof may be altered and others omitted without departing from the general results outlined and the invention within the scope of the appended claims.

What is claimed is:

1. In apparatus for making a prestressed concrete pipe with tensioned wires embedded in the Wall of the pipe and maintaining the pipe longitudinally compressed, the combination comprising a tubular mould casing, a mould end axially abutting one end'of said casing and formed to mould one end of said concrete pipe, a mould end axially abutting the other end of said casing and formed to mould the other end of said concrete pipe, a plurality of highly tensioned high tensile steel wires extending generally longitudinally through the moulding cavity within said tubular casing and through openings in said mould ends to beyond the outer sides of said mould ends, each of said tensioned wires having a section of reduced cross-sectional area adjacent the inner side of the respective mould ends which provides break-oit points for said tensioned wires within the wall of a hardened concrete pipe contained in said moulding cavity, and a plurality of wire-anchoring devices supported by said mould ends at the outer sides of said mould ends whereby the mould ends are pressed against the tubular casing by the pull of said wires, each of. said devices having jaws frictionally gripping one of said tensioned wires, said jaws positionable laterally to the axis of a tensioned wire, and means acting on said jaws for forcing the jaws in gripping engagement with a wire while the Wire gripped thereby is maintained in tension by said devices and said mould ends, each of said devices rotatable about the axis of a gripped tensioned wire with said jaws in gripping engagement with the gripped tensioned wire so that all of the gripped tensioned wires may be broken adjacent the inner sides of the mould ends upon rotation of said devices and the mould ends freed from the lengths of said wires embedded in said pipe.

2. In apparatus for making a prestressed concrete pipe with tensioned wires embedded in the Wall of the pipe, the combination comprising a cylindrical mould having an outer shell and a mould end ring in abutting relationship with each end of said mould shell, and means for I axially constraining said end rings on said shell, said means including a plurality of highly tensioned high tensile steel wires spaced at substantially equal distances around the space within said mould shell and extending through openings in said mould end rings, and individual anchors having jaws in gripping engagement with the respective ends of the wires extending through said openings in said rings and supported against the pull of said wires at the outside of said mould end rings, said anchors having cylindrical noses extending within bores in the outer surface of said end rings and transaxially disposed bearing surfaces for supporting said anchors in opposition to the tension in the gripped'wires, said anchors provided with tool engageable portions extending outwardly be yond the outer surfaces of said end rings whereby the anchors are rotatable with respect to the mould end rings to rotate the gripped portion of a wire while stressed in tension, each of said tensioned wires having a notch adjacent the inside moulding surfaces of said end rings adapted to be broken through upon rotation of said anchors.

, 3. A method for making prestressed pipes or similar tubular bodies of concrete having highly tensioned high tensile steel wires extending longitudinally and in bonded relationship with the concrete wall of the pipe, said method comprising the steps of establishing break-off points of reduced torsional resistance in each high tensile steel wire to be embedded in the wall of the pipe'at such spaced-apart distance as -to locate said break-oh: points within the pipe wall adjacent to each end of the pipe when the wire is stretched under a predetermined tension, mounting the wires in a pipe mould having ends which determine the length of a pipe to be moulded with the wires passing through openings in the opposite ends of the mould, stretching the wires under said predetermined tension and so locating said break-cit points adjacent to but removed inwardly of the mould from the moulding sides of the mould ends, filling the mould with a concrete mix while holding said wires in tension and thereby embedding said tensioned wires in concrete between the mould ends, curing the concrete mix while the wires are strained under said predetermined tension, and after the concrete is cured, breaking said tensioned wires in the concrete adjacent to the inner sides of said mould ends by individually twisting the portions of said wires outside of said mould ends while the wires are stretched in tension, and then removing the mould ends from the hardened concrete pipe.

4. In the making of a concrete pipe with pretensioned high tensile steel wires extending longitudinally in the concrete wall of the pipe and maintaining the wall longitudinally compressed, the method which comprises establishing break-off points in high tensile steel wires by locally reducing the torsional strength of each wire, prior to its stretching, at two locations along the wire disposed apart at such a distance as to locate said points within the concrete of the pipe adjacent to, but inside of, the respective ends of the pipe when the wire is stretched under a predetermined tension for holding the concrete wall of the pipe compressed, said predetermined tension exceeding a stress of about 100,000 pounds per square inch, arranging the unstretched wires in a pipe mould having end walls which determine the length of a concrete pipe to be moulded with the ends of the wires protruding from the opposite end walls of the mould, gripping the ends of the wires outside of the mould, subjectthe wires to said predetermined tension by applying forces to the gripped ends and concomitantly locating said break-oh. points adjacent to the end walls of the mould but between the end walls when the wires are stretched, placing a quantity of a concrete mix into the mould sutficient to form a concrete pipe of a length determined by the walls of the mould and thus embedding the lengths of said tensioned wires between the end walls of the mould in the concrete mix, allowing the concrete to harden while maintaining the wires tensioned by the continued application of said forces to the gripped ends of the tensioned wires, and discontinuing the application of said forces by twisting the gripped ends of the tensioned Wires while they are still tensioned and until they break at the break-E points and thereby transferring the reaction of the tensioned wires to the hardened concrete.

5. In a method according to claim 4 wherein said break-off points are spaced apart along the tension-free wires a distance S approximately equal to L-2d 1+5 E wherein L stands for the inside length of the mould between mould ends, a stands for the distance of the location of a break-off point from an adjacent end of the mould after the wires are stretched under said predetermined tension, p stands for the intensity of tensile stress in the wires, and E is the modulus of elasticity of the wires.

6. In the forming of a concrete pipe having highly tensioned high tensile steel wires extending longitudinally and in bonded relationship with the concrete wall of the pipe, the method which comprises reducing the crosssectional area of each of a plurality of tension-free high tensile steel wires at two points along each wire by impressing notches spaced apart at such a distance as to locate the reduced cross-sectional areas of each wire adjacent to, but inside of the respective mould ends of a pipe mould for moulding a given length of a concrete pipe when the wireis stretched under a predetermined tension of a magnitude sufiiciently high to result in holding the concrete wall of the pipe compressed, threading the ends of a plurality of similarly notched wires through openings in said mould ends, anchoring the wires to one of said mould ends with one of the notches of each wire spaced a short distance inwardly from the inside moulding surface of said mould end, stretching each of the wires by applying forces to the ends of each wire of sufiicient magnitude to stress each wire in excess of about 100,000 pounds per square inch and thereby locating the second of said notches of a wire a short distance inwardly from the inside moulding surface of the other mould end, anchoring the stretched wires against said other mould end, filling the mould space between said mould ends with a concrete mix and thereby embedding the stretched wires between the mould ends, curing the concrete, and trans ferring the reaction to the tension in each wires from the mould .ends to the hardened concrete by twisting oil the anchored ends of the tensioned wires at said notches.

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